Dielectric resonator mode suppressor



1970 M. A. GERDINE 3,58,38

DIELECTRIC RESONATOR MODE SUPPRESSOR Filed March 22 1968 2 Sheets-$heet 1 DIELECTRIC RESONATOR RADIUS m Z MODE PPRESSING SSY MATERIAL FIG. 1 [8 FIG. 2A

I H Eq) LINES iijff g O L z t I2 '64 L29 2 MAGNITUDE(HZORE) l/E/VTOR A. GED/[V5 A T TOP/VEV United States Patent 3,548,348 DIELECTRIC RESONATOR MODE SUPPRESSOR Milton A. Gerdine, Eatontown, N .J assignor to Bell Telephone Laboratories, Incorporated, Murray Hill and Berkeley Heights, N.J., a corporation of New York Filed Mar. 29, 1968, Ser. No. 717,150

Int. Cl. H01p N16 US. Cl. 33398 12 Claims ABSTRACT OF THE DISCLOSURE A high permittivity dielectric resonator disc having a lossy film resistor on its cylindrical surface is positioned within a waveguide at a crossguide location where it couples to the magnetic field of a selected unwanted microwave transmission mode. The circumferential electric field of the resonator produces a current in the lossy resistor and dissipates the coupled energy.

BACKGROUND OF THE INVENTION This invention relates to electromagnetic wave propagation in a hollow waveguide, and more particularly, to the suppression of unwanted modes in cases where the guide is capable of supporting both a desired Wave mode and an unwanted higher order mode.

Multifrequency microwave transmission apparatus using a common antenna and waveguide run is becoming increasingly popular. A single waveguide run capable of supporting a plurality of widely separated frequencies in dominant modes will be dimensioned such that higher order modes of the higher frequencies will necessarily also be supported and the fact that the waveguide run is overmoded may create undesired loss and spurious signals. Thus, an efficient mode suppressor is required to attenuate these unwanted higher order modes.

, Conventional methods of absorbing higher modes include waveguide couplers with lossy terminations, leaky wall filters and lossy material inserted within the waveguide to couple with the electric field of a selected mode. Lossy gyromagnetic material has also been placed within waveguides to selectively couple unwanted modes. As disclosed in A. M. Clogston Pat. 2,948,870, mode selectivity has been created by varying an external magnetic field so as to form within a gyromagnetic material a magnetic field which corresponds to and couples with the field of the unwanted mode.

It is the object of the present invention to provide a passive mode suppressor which will efiiciently and selectively attenuate an unwanted higher order mode at a designated frequency without affecting the corresponding dominant mode and without creating additional mode conversions or reflections.

SUMMARY OF THE INVENTION In accordance with the present invention, selective mode suppression is provided within a hollow waveguide by a suppressor formed from a high permittivity microwave dielectric resonator, such as a cylindrical disc of rutile, which has a lossy film resistor placed upon its cylindrical surface. The resonator has a fundamental TE mode where 6 is defined by its dielectric and geometric properties. If the suppressor is placed coaxially with a magnetic field associated with its resonant frequency, it will couple to that field and produce a circumferential electric field on the cylindrical surface. The coupled energy will be dissipated by the resulting current passing through the lossy film. If such a suppressor is placed within a waveguide at a location where the resonant frequency magnetic field which is uniaxial with the resonator is strong for the selected unwanted mode and relatively weak for the desired mode, then the unwanted mode will be severely attenuated without an appreciable effect upon the desired mode.

The structural advantages of the invention are that it is passive and simple to fabricate and install within a waveguide. Dielectric resonators are small relative to the waveguide cross section and may be positioned with great accuracy. Thus, they may be coupled to a selected mag netic field with precision. The resonant properties afford the further advantage of frequency selectivity which makes the invention extremely useful in communications systems where different information is transmitted in different frequency bands.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a suppressor in accordance with the invention;

FIG. 1A is a graphic representation of magnetic field lines in a longitudinal plane of a suppressor in accordance with the invention;

FIG. 1B is a graphic representation of electric field lines in a transverse section of the suppressor in FIG. 1A;

FIG. 2A is a graph representing the electric field intensity as a function of the axial dimension of a sup- :pressorin accordance with the invention;

FIG. 2B is a graph representing the intensities of the electric field and the magnetic field as a function of the radial dimension of the suppressor in accordance with the invention;

FIG. 3 is a longitudinal plan view of a rectangular waveguide with a graphic representation of magnetic field lines of the TE and TE modes and a suppres sor for attenuating the TE mode;

FIG. 4 is a longitudinal plan view of a rectangular waveguide with a graphic representation of magnetic field lines of the TE and TE modes and a suppressor for attenuating the TE modes; and

FIG. 5 is a transverse section of a rectangular waveguide in partial perspective illustrating an embodiment of the invention wherein a group of suppressors are ar ranged to attenuate the TE and TE modes.

DETAILED DESCRIPTION In order to understand the present invention the properties of the microwave dielectric resonator must be examined. Referring to FIGS. 1, 1A, and 1B, there is shown a cylindrical dielectric resonator 11 with its dominant TE mode electric and magnetic field lines graphically represented. As is shown, the magnetic field H of the resonator 11 is substantially axial and may be represented as H to a first order approximation. The electric field E is primarily circumferential and may be likewise represented as E,,. As can be seen in FIGS. 2A and 2B the strongest magnetic field exists along the axis and the strongest electric field exists on the circumference midway between the ends of the cylinder.

To a first order approximation, the air-dielectric interface for a low loss, high permittivity dielectric resonator can be considered an open circuit boundary. For the assumed open circuit boundary the dielectric resonator is the exact mathematical dual of the perfectly conducting boundary resonator with material parameters 5' and ,u. if e=1 and ,u'=e where e is the dielectric constant of the dielectric resonator. Assuming for example that the dielectric is circular cylindrical in shape, the lowest order resonant TE mode can be equated to the TM mode in a circular perfectly conducting waveguide resonator.

It should be remembered, however, that this is an approximation and resonant fields in a dielectric do vary in the axial direction in contrast to the TM field in a circular perfectly conducting waveguide. The fundamental mode in the dielectric resonator is commonly referred to as the TE where & 6 7r 1 1) The following terms expressed in consistent units are used in Equation 1 and the discussion below:

'y =propagation constant in dielectric waveguide propagation constant in air filled waveguide x =resonant wavelength in free space A =cutoff wavelength of the TE mode in circular cylindrical waveguide L=length of dielectric D=diameter of dielectric e=dl6l6CiriC constant.

The resonant wavelength, A of the TE mode is given by:

from which a D and L can be determined for a specific dielectric constant, e, and selected resonant wavelength A A circular cylindrical resonator formed of a ceramic TiO or rutile (e-l00) is preferred, and a resonant frequency of 11 gHz., for example, will be provided by a cylinder of length, L=0.33 inch, and diameter, D=0.l50 inch. However, other dielectric materials having high dielectric constants on the order of e 80 and small loss tangents, on the order of are also acceptable. (Dielectric loss is, as is well known, proportional to the loss tangent.) It should be clearly understood, however, that the present invention is in no way limited to circular cylindrical dielectrics. Though the relationships above apply to a circular geometry, other relationships define the resonant properties of other geometric configurations.

In accordance with the present invention, it is desired to dissipate the power in a selected propagating mode. Recalling that a low loss dielectric resonator coupled to a propagating mode or a lossy dielectric resonator which produces a substantial mismatch with a propagating mode will give rise to reflections, it is preferred that the resistance of the lossy material be approximately equal to the characteristic impedance of the mode which is to be dissipated. If, for example, the characteristic impedance of the selected mode is approximately 800 ohms at the resonant frequency, then a like value of total dissipative impedance is suggested. An equivalent circuit of the dissipative impedance deposited on the resonator is a tank circuit in parallel with a resistor and at resonance the combination will appear as a resistor alone.

As will be discussed below, suppressors would likely be used in groups and if four suppressors are placed in a single transverse plane they will cooperate as if in series and should each contribute roughly one-quarter or 200 ohms of the total dissipative impedance. As indicated in FIGS. 2A and 2B the electric field is maximum on the cylindrical surface at L/Z. It is therefore preferred that the resistive film 12 be deposited by suitable thin film techniques in narrow circumferential strip about the middle of the resonator 11. Experimental adjustment of the location of the resistor 12 may, however, be needed to optimize heat dissipation.

The resistor should be made of a material and size such that it has no substantial effect upon the electromagnetic field pattern of the dielectric resonator. If, for example, a tantalum-based resistor having a sheet resistance of 5 ohms per square is deposited on the surface of the resonator in a circumferential band 0.015 inch wide a resistance of approximately 200 ohms will result on a resonator having a diameter of 0.150 inch.

The suppressor 16 thus produced will attenuate resonant frequency energy in the mode to which it is coupled. Referring to FIG. 3 where the magnetic field lines are represented in waveguide 15, the maximum longitudinal field of the T E is seen at crossguide positions a/ 3 from each narrow wall 13 and 14, where a is the distance between the 'walls 13 and 14. A suppressor placed at either of these positions and aligned coaxially with the maximum magnetic field component, i.e., parallel to the longitudinal axis of the guide 15, will afford maximum coupling of suppressor 16 to the TE mode. Suppressor 16 can be mounted by any of the numerous Ways known in the art, such as by imbedding it in a low loss material having a dielectric constant close to one. The magnetic field of the TE mode at resonant frequency excites the resonator 15 in the TE mode and the resulting circular electric field forces a current in the lossy resistor 12 thus dissipating the unwanted energy in the TE mode. Similarly, as shown in FIG. 4 a suppressor 16 placed at crossguide position a/ 2 will couple with and dissipate the TE mode. No longitudinal component of the TE mode exists at the a/ 2 position, and therefore the dominant TE mode will not be attenuated by suppressor 16 at that position.

However, referring to FIG. 3 at the one-third crossguide location where the maximum longitudinal component of the magnetic field of the unwanted TE mode exists, a substantially longitudinal component of the desired TE mode also exists and thus some variation of the position from the maximum location may be experimentally re quired in order to optimize operation.

As indicated above it is likely that a group of several suppressors 16 mounted in a single transverse plane are required to sufiiciently attenuate a selected mode. Such a configuration is shown in FIG. 5 for suppressing the TE and TE modes. Because of symmetry a suppressor 16 located as indicated to couple the TE mode will simultaneously couple the TE mode. Thus, each suppressor or suppressor group provides attenuation of both polarizations of a selected mode.

In all cases it is to be understood that the above described arrangements are merely illustrative of a small number of the many possible applications of the principles of the invention. Numerous and varied other arrangements in accordance with these principles may readily be devised by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. A microwave transmission device capable of supporting a desired mode and an undesired mode in propagating electromagnetic wave energy,

a dielectric resonator located in said device in a region of minimum magnetic field of the desired mode and where substantial coupling is produced between said resonator and the magnetic field of said undesired mode, and

lossy material aflixed to said resonator in a region substantially coincident with the maximum electric field of said resonator produced by the magnetic field of said undesired mode whereby the lossy material substantially dissipates the energy of said undesired mode without substantially affecting the propagation of the desired mode.

2. A transmission device as in claim 1 wherein said resonator is small compared to the magnetic field pattern of said selected mode.

3. A transmission device as in claim 1 wherein said resonator is circular cylindrical in shape with said characteristic magnetic field being substantially axial.

4. A microwave device comprising a hollow waveguide capable of propagating a desired mode and an undesired mode of electromagnetic wave energy, and having mounted internally within said waveguide a microwave dielectric resonator at a position and orientation characterized in that said position is in a region of strong magnetic field of the undesired mode and is coincident with a relatively weak magnetic field region of the desired mode, said resonator being aligned such that said strong magnetic field produces a strong resonator electric field which is substantially at a maximum value at one region of said resonator, lossy material aflixed to said resonator at said one region, the orientation of said lossy material aflixed to said resonator being such that the incident electrical energy of the desired mode is substantially unaffected by said material.

5. In a hollow waveguide capable of supporting electromagnetic Wave energy centered about a desired freguency propagating in a desired mode and an undesired mode, a mode suppressor capable of selectively attenuating the undesired mode comprising a dielectric microwave resonator resonant at said desired frequency and capable of producing a characteristic electric field from the magnetic field of said undesired mode, and electrically lossy material afiixed to said resonator in a region substantially coincident with the maximum value of said electric field to dissipate said electric field in said material, said resonator with said lossy material afiixed being positioned within said waveguide in a region where a strong magnetic field of said undesired mode is coincident with a relatively weak magnetic field of said desired mode such that the magnetic field of said resonator is aligned with the magnetic field of said undesired mode, whereby the energy in said undesired mode at said desired frequency is coupled to said resonator and dissipated in said lossy material.

6. A Waveguide device as in claim wherein said resonator is a cylindrical block of material having a dielectric constant greater than 80 and a loss tangent on the order of and wherein said lossy material is a lossy film resistance deposited upon the cylindrical surface of said block.

7. A waveguide device as in claim 5 wherein said resonator is formed of rutile.

8. A waveguide device as in claim 7 wherein said lossy material is tantalum.

9. A waveguide device as in claim 5 wherein said resonator is circular cylindrical in shape with said characteristic magnetic field being substantially axial and wherein said resonator is positioned within said waveguide such that its axis is parallel to the longitudinal axis of said Waveguide.

10. A waveguide device as in claim 5 wherein said waveguide is rectangular and wherein said desired mode is the dominant rectangular TE mode and said undesired mode is the higher order rectangular TE mode, and wherein said resonator is positioned within said waveguide at a cross-guide position approximately one-third the width of said waveguide from a narrow wall of said waveguide.

11. A waveguide device as in claim 5 wherein said waveguide is rectangular and said desired mode is the dominant rectangular TE mode and said undesired mode is the higher order rectangular TE mode and wherein said resonator is positioned within said waveguide at a cross-guide position approximately equidistant from the narrow walls of said waveguide.

12. A microwave transmission waveguide capable of supporting a desired mode and an undesired higher order mode,

mode suppressing means comprising a plurality of dielectric resonators located at positions corresponding to high intensity regions of the magnetic fields of the undesired mode and regions of low intensity magnetic fields of the desired mode, each of said resonators having a circular cylindrical shape with its characteristic magnetic field being substantially axial, said resonators being oriented within said device such that the axes of said resonators are parallel to the longitudinal axis of said waveguide, each of said resonators having a circumferential resistive band in a region substantially coincident with the maximum electric field of said resonator generated by said magnetic fields of the undesired mode, and said resonators having impedance values in said resistive bands such that the cumulative impedance sum of said resistive bands is substantially equal to the characteristic impedance of said undesired mode, whereby the energy of the undesired mode is substantially dissipated among said plurality of resistive bands of the dielectric resonators.

References Cited UNITED STATES PATENTS 3,251,011 5/1966 Unger 333-98 3,300,729 1/1967 Chang 33383X 3,475,642 10/1969 Karp et al. 33383(A)X PAUL L. GENSLER, Primary Examiner US. Cl. X.R. 333-81 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 3) D d December 15,

Inventor(s) Milton A. Gerdine It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 2, column line 7 "selected" should read --undesi:

Claim 3, column 5, line 1, "char-" should be deleted;

line 2, "acteristic" should be deleted,

after "field" -of said undesiri mode-- should be inserted.

Claim 9, column 5, line 50, "character-" should be deleted;

line 51, "istic" should be deleted,

after "field" --of said undesire mode-- should be inserted.

Signed and sealed this L th day of May 1 971 (SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, Attesting Officer Commissioner of Pete: 

